In accordance with the development of high density mounting technology for LSIs, the number of input and output pins is increased relative to an area of semiconductor chips, so that the number of the semiconductor chips mounted on a circuit board is decreased, and the size of the circuit board is made small. Thus, the high density mounting of LSIs is realized by using a mounting method of, for instance, the lead type or the surface mounting type.
A conventional semiconductor package using a method of "LOC(Lead On Chip)" fabricated by the high density mounting technology comprises a semiconductor chip having electrodes thereon, and a lead frame having leads adhesively attached on the semiconductor chip, wherein the leads are bonded to the electrodes by bonding wires, and the semiconductor chip and the bonded leads are sealed to expose an outer portion of the leads by a packaging resin. In this semiconductor package, the attaching of the leads to the semiconductor chip is conducted by using a punched insulating film having thermally activating adhesive layers on the both surfaces thereof. For this purpose, the punched insulating film is sticked to the leads of the lead frame in advance of a fabrication process for the semiconductor package.
Here, a conventional method for sticking an insulating film to a lead frame will be explained.
In the sticking method, an insulating film is punched to provide a punched insulating film by using a punch and a die cooperating with each other. The punched insulating film is applied on a lead frame which is heated on a heater in accordance with the lowering motion of the punch. Thus, the punched insulating film is adhesively attached on an inner lead portion of the lead frame.
In the LOC type semiconductor package, the punched insulating film is subject to thermal steps such as the wire bonding, the resin sealing, etc. For this reason, the punched insulating film and adhesive layers on the both surfaces must be heat-proof at a temperature ranging 250.degree. to 400.degree. C.
In an experiment similar in condition to the conventional method for sticking an insulating film to a lead frame, when a heat cycle in which a punch having a length of 18.5 mm is heated at a temperature of 400.degree. C. for one and half seconds, and is then cooled for one second is conducted, it is detected in the analysis result by FEM (Finite Element Method) that the punch is burnt with a die in accordance with the thermal expansion of the punch.
Therefore, a clearance is properly set between the punch and the die in the conventional method for sticking an insulating film to a lead frame to avoid the burning of the punch with the die.
In addition to the application of a punched insulating film to the LOC type semiconductor package, it is also used for suppressing the difference of position levels at a tip portion of leads of a lead frame and the position shift of the leads. For this purpose, the same method as the above described method for sticking an insulating film to a lead frame is used, wherein a punched insulating film is not required to be heat-proof at a temperature of 250.degree. to 400.degree. C., but only to have a heat-proof resistance of approximately 150.degree. C., because the punched insulating film is not used for insulating the lead frame from the semiconductor chip.
In the conventional method for sticking an insulating film to a lead frame, however, there is a disadvantage in that an insulating film is not properly punched to result in deformed edges (burrs and chips) on a punched insulating film, when the punch has a length greater than 13 mm to arise the remarkable influence on a clearance between the punch and the die due to the thermal expansion of the punch. In order to overcome the disadvantage, the punch and the die must be designed to provide a proper clearance therebetween in the thermally expanded state, and the punch must be pre-heated prior to operation.
Further, there is a disadvantage in that a pressing force of the punch on the lead frame is large to deform the punched insulating film, and, in a rare case, the lead frame, as described in detail later.